This invention relates to gas detection and measurement, and more particularly to an apparatus for measuring gas concentration of a selected gas in a fluid.
Spectrochemical analysis includes a number of techniques for determining the presence or concentration of elemental or molecular constituents in a sample through the use of spectrometric measurements. One particular technique, spectrophotometric analysis, is a method of chemical analysis based on the absorption or attenuation of electromagnetic radiation of a specified wavelength or frequency. A spectrophotometer for providing such analysis generally consists of a source of radiation, such as a light bulb; a monochromator containing a prism or grating which disperses the light so that only a limited wavelength, or frequency range is allowed to irradiate the sample; the sample itself; and a detector, such as a photocell, which measures the amount of light transmitted by the sample.
The near ultraviolet spectral region from 200 to 400 nm is commonly used in chemical analysis. An ultraviolet spectrophotometer usually includes at least a lamp as a radiation source, a sensor and appropriate optical components. Simple inorganic ions and their complexes as well as organic molecules can be detected and determined in this spectral region.
In most quantitative analytical work, a calibration or standard curve is prepared by measuring the absorption of a known amount of a known absorbing material at the wavelength at which it strongly absorbs. The absorbance of the sample is read directly from the measuring circuit of the spectrophotometer.
Most gases have at least one well-defined peak of absorption at a certain wavelength. Ozone (O.sub.3), for example, has one peak of absorption at 253.7 nm, in the ultraviolet range of the spectrum. The concentration of a selected gas in a sample can be obtained by solving an equation, known as the Beer-Lambert equation as follows: EQU I.sub.s =I.sub.r *e.sup.-.di-elect cons.LC
Where:
I.sub.s is the intensity of light from the sample; PA2 I.sub.r is the intensity of light from the reference; PA2 .di-elect cons. is the ozone absorption coefficient constant at the wavelength used; PA2 L is the length of the absorption chamber (path length of the light); and PA2 C is the concentration of gas in weight/volume.
Since L and .di-elect cons. are fixed quantities, gas concentration can be determined by measuring the intensities I.sub.s and I.sub.r. The Beer-Lambert equation provides an absolute determination of gas concentration. The relationship requires the measurement of a "reference" light intensity and a "sample" light intensity.
Most gas analyzers currently employed for process gas measurement, are fed with gas from a small gas sidestream; and it is the gas from the sidestream that is analyzed. However, the diversion of gas can be wasteful. For example, with respect to ozone generators a portion ozone output from the generator is diverted to the analyzer and subsequently directed to a scrubber or gas neutralization/destruction device. It would be desirable to eliminate such a diversion and to directly sample much of or the entirety of the generator output. This would increase the effective output of the generator and eliminate the requirement for a scrubber. However, known ultraviolet analyzers are functionally limited to very small test cells that are unable to accommodate significant flow volumes.
An additional problem with respect to known light absorption analyzers is that in order to measure the concentration of a given gas, the analyzer requires a "zero gas present" reference, or a zero reference, to compare with the gas stream. One approach for providing a zero reference involves using a beam splitter to divert light away from an absorption cell and an associated sensor for measurement by a second sensor. However, as the optical components exposed to the gas stream are gradually soiled to varying degrees during usage and the optical components associated with the diverted light are not soiled, a drift between the two measurements develops which becomes progressively more inaccurate over time.